The present invention relates to improvements to an electrostatic copying machine.
A problem existing in prior art electrostatic copying machines utilizing a dry powdered developing substance comprising carrier particles and toner particles is determining the condition of the developing substance and replacing the same when it has deteriorated to an excessive extent. Since only the toner substance is consumed during the developing process, it is common practice to recycle the carrier particles and unused toner particles and add fresh toner particles to maintain the toner density, or the ratio of toner particles to carrier particles, at a constant value. However, prolonged reuse of the carrier particles causes degeneration or deterioration thereof and a consequent decrease in the developing ability of the developing substance. Such deterioration produces a reduction in toner image density and copies having a washed-out appearance. For this reason, it has further been common practice to replace the developing substance when a decrease in image density is visually observed or after the developing substance has been used to produce a predetermined number of copies.
This problem is complicated by the fact that the toner image density is effected by factors other than deterioration of the developing substance. Dirt accumulation or tarnish on the various optical components of the copying machine cause a reduction of the liquid image intensity incident on a photoconductive drum or the like of the machine and resultant variation in toner image density. Deterioration of the dielectric and photoconductive properties of the drum also cause variation in toner image density.
It is therefore impossible to determine the codition of the developing substance merely by observing the density of the copies produced by the machine. Good developing substance may be wasted by premature replacement and excessively deteriorated developing substance may be used too long.
Another problem which has existed heretofore in electrostatic copying machines is optimally controlling the exposure and developing bias voltage as a function of deterioration of various copying machine elements and the density or color of the background areas of original documents. Excessive exposure and bias voltage produced washed-out copies, or copies of insufficient density. In sufficient exposure and bias voltage produce copies with gray background areas.